1. Field of the Invention
This invention relates to cables having a helical element, including coaxial cables, multipair cables, or lightguide cables for telecommunications use.
2. Description of the Prior Art
Cables having one or more helical elements are used in a variety of applications, including telecommunications cables, such as coaxial cables, multipair cables, or lightguide cables. A helical wrap can serve as an armor member of such a cable when located near the outer portion of the cable. Other helical wrap elements include tapes, plastic ribbons, binders, etc. A helical wrap of a conductive material may also be used in the transmission of electrical power or electrical information.
Unless steps are taken to prevent it, a helical element typically introduces a torque response to a cable. That is, when an axial force is applied to the cable, causing it to elongate, there is a tendency of the cable to twist. Depending upon the construction of the cable, this twist may be either in the direction that causes the helical wrap to tighten, or in the direction that causes it to unwind. Such a torque response can in some cases cause a cable to kink when a cable is being unwound from a drum or otherwise handled, as for example during installation of the cable. Other handling difficulties can also result from a torque response. The tightening or unwinding of the helical element can also adversely affect the performance of the cable.
It is possible to counteract the torque response of one or more helical layers by providing for an opposite torque response in another helical layer. For example, one helical layer wound in one direction can be counteracted by an overlying helical layer wound in the opposite direction. Techniques for analyzing the torque response of helically wrapped cables have been developed which allow for predicting the lay angle of a counteracting helical element; see, for example, "Mechanical Characterization of Cables Containing Helically Wrapped Reinforcing Elements", by T. C. Cannon and M. R. Santana in the Proceedings of the 24th International Wire and Cable Symposium (1975), Cherry Hill, N.J.